(De)multiplexer with four &#39;F&#39; configuration and hybrid lens

ABSTRACT

An apparatus for multiplexing or demultiplexing optical signals in an optical communication system includes a plurality of optical waveguides aligned generally along the same optical axis each having a propagating end. A reflective grating is optically coupled to the plurality of optical waveguides along the optical axis and has a surface receiving an optical signal emitted from at least one of the optical waveguides. The surface diffracts the optical signals into at least one other of the optical waveguides. A collimating/focusing optic having a select focal length is optically coupled between the plurality of optical waveguides and the reflective grating along the optical axis. The collimating/focusing optic is positioned relative to the propagating ends of the plurality of optical waveguides and the reflective echelle grating to propagate a telecentric optical beam(s) into the at least one other of the optical waveguides.

RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional PatentApplication Serial No. 60/272,748, filed Mar. 1, 2001, entitled “EchelleGrating (De)Multiplexer with Four “F” Configuration and Hybrid Lens” andis a continuation-in-part application of U.S. patent application Ser.No. 09/628,774, filed Jul. 29, 2000, entitled “Echelle Grating DenseWavelength Division Multiplexer/Demultiplexer.” application Ser. No.09/628,774 claimed priority from U.S. Provisional Patent ApplicationSerial No. 60/209,018, filed Jun. 1, 2000, entitled “Lens-coupledWavelength Division (De)multiplexing System Utilizing an EchelleGrating;” No. 60/152,218, filed Sep. 3, 1999, entitled “Method andApparatus for Dense Wavelength Multiplexing and De-multiplexing FiberOptic Signals Using an Echelle Grating Spectrograph;” No. 60/172,843,filed Dec. 20, 1999, entitled “Improved Method and Apparatus for DenseWavelength Multiplexing and De-multiplexing Fiber Optic Signals Using anEchelle Grating Spectrograph;” and No. 60/172,885, filed Dec. 20, 1999,entitled “Method and Apparatus for Dense Wavelength Multiplexing andDe-multiplexing Fiber Optic Signals from a Single or Many IndividualFibers Using a Single Echelle Grating Spectrograph,” each of which isincorporated herein in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to fiber-opticcommunications, and more particularly to bulk grating (de)multiplexershaving a variable pass band and telecentricity.

BACKGROUND OF THE INVENTION

[0003] Described below with reference to FIGS. 1-6 is a bulk echellegrating DWDM device. One significant advantage of this echelle gratingDWDM device is relatively high angular dispersion of the grating,enabling a (de)multiplexer having a compact footprint along the opticalaxis. However, this angular dispersion raises other design issues thatinhibit efficient coupling of light from a fiber carrying a multichannelsignal to an array of single channel fibers.

[0004]FIG. 16 is a schematic representation of an echelle grating(de)multiplexer 210, which is described in greater detail with referenceto FIGS. 1-6 below. FIG. 16 includes only seven single channel outputwaveguides or fibers and these are shown equally spaced for ease ofillustration. In fact, there can be forty or more single channel fibersassociated with the multichannel fibers and the spacing between fibersmay increase slightly from top to bottom. In a (de)multiplexing mode,light is emitted from a multi channel input fiber 212 (which typicallyis the first or last fiber in the plane of the single channel fiberarray 220) forming a cone or beam of light 214 which is collimated bythe lens 216 and directed on the reflective grating 218 which dispersesthe multichannel signal into its constituent channels according towavelength. Thereafter single channel light reflected from the grating218 is focused by the lens 216 to corresponding single channel outputfibers 220. For channels which are directed to the most distal singlechannel fibers, for example fiber 222, the focused cone of light 224 hasan “axis” 226 which is at a significant angle with respect to the face228 of the distal fiber 222. Because light is coupled most efficientlyto the output fibers along an axis perpendicular to the face 228, thegreater the angle the axis 226 forms with respect to the face 228 theless efficient the coupling. Because of the relatively high angulardispersion of echelle gratings, the angle of the axis 226 in an echellegrating (de)multiplexer can be great enough to cause significant losswhen coupling with the most distal output fiber 222.

[0005] The preferred way of addressing this problem is to provide atelecentric (de)multiplexer. “Telecentricity” is a property of theimaging system having effectively an exit pupil at an infinite distancesuch that the angle of the axis 226 is normal to the plane of the face228 of the single channel fibers 220. A telecentric optical imagingsystem also ensures that the transmission efficiencies are the same inthe multiplexing or demultiplexing mode of operation, i.e., abidirection device. One way to provide telecentricity is to replace thesingle lens 216 (or a two piece lens described below with reference toFIGS. 1-6) with a telecentric triplet lens. However, this requires theintroduction of additional components which increases (de)multiplexersize, weight, complexity and cost.

[0006] One additional problem with the (de)multiplexer configurationillustrated in FIG. 1 is that the multichannel fiber 212 and the singlechannel fibers 220 have relatively narrow wavelength pass bands. Thepass band is determined by the diameter of the cores of the opticalfibers 212 and 220, and the spacing between adjacent fibers in array220. This makes it difficult to provide for efficient coupling of lightto the respective fibers, particularly for high speed data rates.

[0007] Martin, U.S. Pat. No. 6,084,695, describes a structure forwidening the effective pass band of the fibers. In essence, Martinteaches providing a microlens array consisting of a gradient index lens(“GRIN”) microlens aligned with each fiber. Martin requires that thepitch between adjacent microlenses of the array be the same (within±0.23 μm) as the pitch between corresponding adjacent optical fibers.Furthermore, because Martin teaches the use of GRIN microlenses, thestructure taught by Martin requires that the face of the fibers beprecisely at the focal plane of the microlenses of the array. The tighttolerances required by these alignment demands render the structuretaught by Martin difficult and expensive to build, and subject to lossesin efficient coupling due to any imperfections in component fabricationand changes in the alignment caused by environmental factors such astemperature changes, vibrations or the like. In addition, the structureof Martin does not allow for adjustability or tunability of the passband. Furthermore, Martin fails to teach or suggest the advantages of atelecentric configuration.

[0008] A. W. Lohmann (1996) Optical Comm. 86:365, as well as S.Sinzinger and Jahans, “Microoptics” Wiley-VCH, p.215-217; 221-222 (1999)describe a 4F setup for imaging an input array into an output array andnotes that the 4F imaging configuration is telecentric. in section 7.2.4Sinzinger and Jahans describe hybrid imaging, which consists of imaginglenses in combination with an array of microlenses, with the microlensesreducing beam divergence, thus reducing the size and complexing of theconventional imaging lens. However, Lohmnann, Sinzinger and Jahans werediscussing optical cross connects and didn't recognize the significantadvantages the hybrid lens 4F configuration would have in combinationwith a reflective echelle grating (de)multiplexer and its high angulardispersion.

[0009] The present invention is directed toward overcoming one or moreof the problems discussed above.

SUMMARY OF THE INVENTION

[0010] A first aspect of the present invention is an apparatus formultiplexing or demultiplexing optical signals in an opticalcommunication system. The apparatus includes a plurality of opticalwaveguides aligned generally along the same optical axis each having apropagating end. A reflective grating is optically coupled to theplurality of optical waveguides along the optical axis and has a surfacereceiving an optical signal emitted from at least one of the opticalwaveguides. The surface diffracts the optical signals into at least oneother of the optical waveguides. A collimating/focusing optic having aselect focal length is optically coupled between the plurality ofoptical waveguides and the reflective grating along the optical axis.The collimating/focusing optic is positioned relative to the propagatingends of the plurality of optical waveguides and the reflective echellegrating to propagate a telecentric optical beam(s) into the at least oneother of the optical waveguides. In one embodiment, thecollimating/focusing optic is optically coupled to the propagating endsof the waveguide at about the select optical focal length from thecollimating/focusing optic and a point on the surface of the reflectiveechelle grating is spaced the selective focal length from thecollimating/focusing optic. The reflective grating may be an echellegrating having a groove spacing of between about 50-300 grooves permillimeter and a blaze angle of between about 51-53 degrees. In anotherembodiment the reflective grating is an echelle grating having a groovespacing of between about 50-300 grooves per millimeter and a blaze angleproviding an angle of dispersion of between about 0.091 and 0.011degrees per nanometer for a select order of diffraction of between 4-7.

[0011] Another aspect of the present invention is an apparatus for usein optical communication systems to multiplex or demultiplex an opticalsignal. The apparatus includes a plurality of optical waveguides alignedgenerally along the same optical axis, each waveguide having apropagating end. The plurality of optical waveguides include at leastone multiplexed waveguide propagating a plurality of multiplexedchannels and the others of the optical waveguides are single channelwaveguides propagating single channels. A reflective grating isoptically coupled to the plurality of optical waveguides along theoptical axis and receives an optical signal emitted from at least one ofthe optical waveguides and diffracts the optical signal(s) into at leastone other of the optical waveguides. A collimating/focusing optic havinga select focal length is optically coupled between the plurality ofoptical waveguides along the optical axis. The reflective echellegrating is spaced about the select focal length from the reflectiveechelle grating. An array of microlenses resides in opticalcommunication between the propagating end of the single channelwaveguides and the collimating focusing optic with the array ofmicrolenses being positioned relative to the collimating/focusing opticto propagate a telecentric optical beam(s) into at least one other ofthe optical waveguides. In one embodiment, the array of microlenses isspaced at or near a focal plane spaced the select focal distance fromthe collimating/focusing optic. The single channel optical waveguidesand the array of microlenses may be in a linear array. The reflectivegrating may be an echelle grating having a groove spacing of betweenabout 50-300 grooves per millimeter and a blaze angle of between about51-53 degrees. The focal point of at least one microlens may be in acorresponding waveguide. In addition or alternatively, the focal pointof at least one microlens may be at the propagating end of acorresponding waveguide. Preferably the reflective grating is spaced adistance from the collimating/focusing lens such that a point on thereflective surface of the grating is spaced the select focal length fromthe collimating/focusing lens. The point on the reflective surface onthe grating is preferably at about the center of the reflective surfaceof the grating.

[0012] The (de)multiplexer of the present invention having acollimating/focusing optic positioned to propagate a telecentric opticalbeam into the waveguides provides enhanced efficiency in propagatingoptical signals. The telecentric properties of the (de)multiplexer alsofacilitates looser tolerances in the construction and alignment of the(de)multiplexer components. Those embodiments adding an array ofmicrolenses in optical communication with the optical fibers provide fora broader pass band and therefore further facilitate optical coupling ofbeams to the waveguide. Furthermore, the use of the microlens arraydiminishes the necessity for extremely tight tolerances in alignment ofthe components, which greatly assists in the efficient manufacture ofthe (de)multiplexer. The telecentric (de)multiplexers disclosed hereinhave greatest advantage with highly dispersive gratings such as echellegratings described herein. Absent the telecentric properties of the(de)multiplexers coupling efficiency, particularly with the most distalfibers, may be seriously compromised. Those embodiments including thecombination of the telecentric (de)multiplexer and the array ofmicrolenses combine all these advantages to result in a highly efficient(de)multiplexer which can be efficiently manufactured with significantcost savings over prior art devices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic plan view of a multiplexer/demultiplexerusing a bulk echelle grating in accordance with the present invention;

[0014]FIG. 2 is an enlarged cross-section of the echelle grating groovesillustrating relevant dimensions;

[0015]FIG. 3 is a graphical representation of possible step widths andriser heights at different orders which may yield a working echellegrating;

[0016]FIG. 4 is a schematic representation of an example of amultiplexer/demultiplexer with a bulk echelle grating in accordance withthe present invention;

[0017]FIG. 5 is a partial cross-sectional view of a pigtail template;

[0018]FIG. 6 is a perspective view of the multiplexer/demultiplexer withbulk echelle grating of FIG. 1 illustrating the potential adjustment ofthe components;

[0019]FIG. 7 is a schematic view of a first alternate embodiment of themultiplexer/demultiplexer using a bulk echelle grating including a pairof collimating/focusing concave mirrors;

[0020]FIG. 8 is a second alternate embodiment of themultiplexer/demultiplexer of FIG. 7 further including a prism providingfor wavelength dispersion in a horizontal direction;

[0021]FIG. 9 is a third alternate embodiment of themultiplexer/demultiplexer using a single collimating/focusing mirror;

[0022]FIG. 10 is a fourth alternate embodiment of themultiplexer/demultiplexer in accordance with the present invention usingan off-axis parabolic mirror as the collimating/focusing optic with thedevice arranged in a near-littrow configuration;

[0023]FIG. 11 is a fifth alternate embodiment of themultiplexer/demultiplexer of the present invention using a concaveechelle grating;

[0024]FIG. 12 is a schematic representation of an apparatus for dividinga broad bandwidth into bandwidth segments formultiplexing/demultiplexing;

[0025]FIG. 13 is a schematic representation of the embodiment of FIG. 12using three waveband dividing elements; and

[0026]FIG. 14 is a schematic elevation of a pigtail harness having aone-dimensional input array of fibers and a two dimensional output arrayof fibers;

[0027]FIG. 15 is a schematic representation of amultiplexer/demultiplexer having stacked multiplex fibers and atwo-dimensional array of single channel fibers;

[0028]FIG. 16 is a schematic plan view of a prior art bulk opticmultiplexer/demultiplexer;

[0029]FIG. 17 is a schematic plan view of a multiplexer/demultiplexer ina “4F” configuration in accordance with the present invention; and

[0030]FIG. 18 is a schematic plan view of a multiplexer/demultiplexerhaving a “4F” configuration and microlens array in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] A multiplexer/demultiplexer (a “(de)multiplexer”) for use inoptical communication systems 10 of the present invention is illustratedschematically in FIG. 1. It includes a pigtail harness 12 consisting ofan input waveguide 14, a plurality of output waveguides 16 arranged in alinear array adjacent the input fiber, a collimating/focusing lens 18and an echelle grating 20, each of which are optically coupled. In thepresent discussion the (de)multiplexer will be discussed in terms of ademultiplexer. The description applies equally to a multiplexer, onlywith the function of the input and output waveguides 14, 16 reversed.Also, for the sake of clarity, only seven output waveguides areillustrated (the center output waveguides underlies the input fiber inFIG. 1 as can be seen with respect to elements 142 and 148 of FIG. 14).Furthermore, the waveguides 14, 16 are preferably single mode opticalfibers. As will be discussed in greater detail below, in the preferredembodiment, 90 or more output waveguides can be associated with a singleinput waveguide, depending upon the bandwidth channel, separation and anumber of other factors.

[0032] As used herein, “optically coupled” or “optically communicates”means any connection, coupling, link or the like, by which opticalsignals carried by one optical element are imparted to the “coupled” or“communicating” element. Such “optically communicating” devices are notnecessarily directly connected to one another, but may be separated by aspace through which the optical signals traverse or by intermediateoptical components or devices.

[0033] As illustrated in FIG. 1, the (de)multiplexer 10 is in “nearlittrow configuration,” meaning that the incident beam λ_(1−n) and thechannels diffracted off the surface of the grating λ₁, λ₂, λ₃, λ₄, λ₅,λ₆, λ₇ are generally along the same optical axis (that is, they trace avery close path) and the lens both collimates the input beam λ_(1−n) andfocuses the diffracted channels λ₁-λ₇ to the output fibers 16.

[0034] The echelle grating 20, like other gratings such as echellettegratings, uses interference between light wavefronts reflected fromvarious portions of its ruled surface or steps 22 to divide the incidentbeam consisting of a plurality of channels λ_(1−n) having a selectchannel spacing within a select wavelength range λ_(1−n) into separatechannels of wavelength beams λ₁-λ₇ which are angularly dispersed by thegrating into output waveguides some distance away. Referring to FIG. 1,the channel separation of the device (D), which is the product of thefocal length of the focusing/collimating optic the angular dispersionand the incremental channel spacing, is equal to the distance S betweenthe center of adjacent output waveguides. The echelle grating 20 isparticularly suited to use in optical communication systems because of aunique combination of properties: 1) it provides clear channelseparation notwithstanding channels being closely spaced (0.4 nm orless); 2) it provides large spatial separation of channels overrelatively short distances; and 3) it is highly efficient in the rangeof optical communications wavelengths.

[0035] Referring to FIG. 2, for the purpose of this specification,echelle gratings are a special grating structure having groove density(1/d) of under 300 grooves/mm and a blaze angle θ_(b) of greater than45° which typically operate at an order of diffraction greater than 1.In combination, these features enable a multiplexer/demultiplexer thatefficiently separates closely spaced channels over a relatively smallfocal length (e.g., 5 inches) enabling a small form factor form factor(on the order of 10 inches in length or less).

[0036] Consideration of certain external and performance constraintspoint to the desirability of echelle gratings for DWDM. The externalconstraints include the following:

[0037] 1) Minimize focal length, with a focal length of under 6 inchesdesired.

[0038] 2) Center wavelength in near infrared, approximately at thecenter of the C-band, 1550 nm.

[0039]3) A minimal channel spacing (e.g., 0.4 nm or less).

[0040] 4) Large free spectral range, 150 nm.

[0041] 5) System f number in the range of 4-8.

[0042] 6) Rugged, minimum cost system.

[0043] The performance constraints include:

[0044] 1) Resolution greater than 20,000.

[0045] 2) High dispersion.

[0046] 3) Flat response across spectral range.

[0047] 4) High efficiency or low loss, (>75%).

[0048] 5) Minimize polarization dependent loss.

[0049] The external constraints of ruggedness size and cost minimizationas well as performance constraints of ease of alignment and highefficiency dictate a littrow configuration, which simplifies the systemoptimization analysis.

[0050]FIG. 2 illustrates the echelle grating geometry and the variablesset forth below.

[0051] θ_(b)=blaze angle

[0052] α=incident angle

[0053] β=diffracted angle

[0054] In littrow, θ_(b)=α≡β

[0055] b=step (reflective surface) size

[0056] d=1/groove density

[0057] a=riser size

[0058] Examination of a number of constraining factors discussed aboveillustrate the utility of echelle gratings for DWDM.

[0059] 1. Constraining Factors: f number (f) in range of 4-8 andresolution (“R”)>20,000.

[0060] Result: For a grating in littrow configuration,${R > {2( \frac{W}{\lambda} )}},$

[0061] where W is the illuminated width of the grating. Thus, orW≈(20,000/2)(1550 nm) or W≈1.55 cm

[0062] W×f=fl (focal length), or

[0063] fl≈1.55 cm×8≈124

[0064] 2. Constraining Factors: Fl>124 mm and channel separation atleast 80μ.

[0065] Result: For an echelle grating in littrow, dispersion${( \frac{\theta_{b}}{\lambda} ) = {{\frac{m}{b} \cdot f}\quad l}},$

[0066] where m=order of diffraction. Thus, assuming channel separationto be at least 80 μ, Δλ=4×10⁻⁴μ and fl=1.2×10⁴μ, m>1.5b.

[0067] 3. Constraining Factors: FSR (free spectral range)>150

[0068] Result: ${{FSR} = \frac{\lambda}{m}},$

[0069] which implies ${m = \frac{1550}{10}},$

[0070] or m≦10.

[0071] 4. Constraining Factors: Wish to provide a flat response over thebandwidth.

[0072] Result: The diffraction envelope must have a broad enough maximumso that loss is minimized at the extremes of the wavelength range. Thisdictates b<8.5μ. An order over 7 spreads the light too much across thediffraction peak, resulting in unacceptably low efficiency. Thus: b<8.5μand m≦7.

[0073] 5. Constraining Factors: High efficiency. (>85°)

[0074] Result: Efficiency is a function of step size. A step size mustbe selected providing a channel width capturing 90% of the signal at aselect order. b>3μ yields suitable efficiency.

[0075] 6. Constraining Factors: Limitations on m from 4. and 2. above.

[0076] Result: 1.5<m<7.

[0077] 7. Constraining Factors: For an echelle grating in littrow mode:$a = {\frac{m\quad \lambda}{2}.}$

[0078] Result: a=

[0079] 3.88μ at m=5

[0080] 4.65μ at m=6

[0081] 5.43μ at m=7

[0082]FIG. 3 illustrates that these constraints and results provide arange of values for a and b at a given range of suitable orders (m).Simulations aimed at maximizing efficiency and minimizing polarizationdependent loss optimize around blaze angles and groove frequencies thatfall in the range of echelle gratings, i.e., 45<θ_(b)<78° and d<300grooves/mm. Furthermore, limitations on manufacturing further dictatethat only echelle gratings can provide the necessary results within theexternal and performance constraints.

[0083] In designing a functioning (de)multiplexer, a number of designparameters were selected that were dictated by many of the external andperformance constraints set forth above. An exemplary configuration isillustrated schematically in FIG. 4, with like elements having the samereference number as FIG. 1. The dictating constraints and their effecton the exemplary bulk echelle grating DWDM are as follows:

[0084] 1. Channel Characteristics

[0085] Currently optical communications utilize what is know as the “C”band of near infrared wavelengths, a wavelength band ranging from1528-1565 nanometers (nm). This provides a bandwidth or free spectralrange of 37 nm available for channel separation. Known prior art(de)multiplexers require a channel spacing of 0.8 nm or even 1.6 nm,resulting in a possibility of only between 48 and 24 channels. Becauseechelle gratings provide markedly superior channel dispersion, a muchsmaller channel spacing of 0.4 nm was chosen, resulting in a possibilityof 93 channels over the C band. As the tuning range of semiconductorlasers increases and optical communications expand beyond the “C” bandto include the “L” band (1566-1610 nm) and the “S” band (≈1490-1527 nm),a total bandwidth of about 120 nm or more is foreseeable, creating apossibility of the (de)multiplexer accommodating 300 channels or moreper input fiber.

[0086] Current optical communications operate primarily at a channelfrequency of 2.5 GHz, known as OC48. At OC48 the channel width λ₄₈ =0.02nm. Optical communications are currently beginning to adopt a frequencyof 10 GHz, know as OC192. At OC192 the channel width λ₁₉₂=0.08 nm.

[0087] 2. Fiber Dimensions

[0088] Standard single mode optical fiber used in optical communicationstypically have an outer diameter of 125 microns (μ) and a core diameterof 10μ. Optical fibers having an outer diameter of 80μ and core diameterof 8.3μ are available, model SM-1250 manufactured by Fibercore. In thisexample, both the input fiber 14 and the output fiber 16 are single modeand share the 80μ outer diameter. Assuming the output fibers 16 areabutted in parallel as illustrated in FIG. 4, this results in the corecenters being spaced 80μ, or a required channel separation D of 80μ atthe select focal length. Because fibers of different outer diameter areavailable and fibers cladings can be etched away, it is possible thatthe 80μ spacing can be reduced, with core spacing of 40μ or less beingforeseeable, which could enable shorter focal lengths or differentechelle grating designs having lesser angular dispersion. The spread ofthe beam emitted from the fiber was 10° at the e-folding distance,although it was later found to be 14° at the 1% point.

[0089] 3. Form Factor

[0090] The design was intended to provide a high channel density in aform factor consistent with or smaller than used in current(de)multiplexer devices. A total length of between 10-12 inches was thedesign target. To accommodate all the optics and harnesses, a maximumfocal length of 5 inches (127 mm) was chosen. As discussed above, inlight of the constraining factors of the f number between 4-8 and aresolution (R)>20,000, a focal length of 124 was ultimately dictated.

[0091] 4. Dispersion Limitations

[0092] In order to prevent the loss of data, it was necessary that thedispersion of the echelle grating be constrained. The initial 0.4 nmchannel spacing at the echelle grating was required to be about 80μ ofseparation at the output fibers (corresponding to the core spacing). Onthe other hand, the 0.08 nm channel width of OC192 frequencies could notdisperse to much greater than the fiber core aperture over the focallength. Thus:${\frac{{channel}\quad {separation}}{{fiber}\quad {spacing}} < \frac{{.4}\quad {nm}}{80µ}},{{{while}\quad \frac{{channel}\quad {width}\quad ( {{OC}\quad 192} )}{{core}\quad {diameter}}} > \frac{0.08\quad {nm}}{8.3µ}}$

[0093] 5. Grating Design

[0094] The variables affecting grating design are:

[0095] 1) wavelength range

[0096] 2) efficiency

[0097] 3) dispersion (D)

[0098] 4) desired resolution$( \frac{\lambda}{\Delta \quad \lambda} )$

[0099]FIG. 3 is a cross-section showing the principle echelle gratingdimensions including: blaze angle (θ_(b)), wavelength range and groovedensity (d).

[0100] For design of the grating, 150 channels centered on 1550 nm waschosen. This results in a physical size of the spectral image of (numberof channels)×(maximum separation, or 150×80μ=12,000μ. This desire tohave 90% of the intensity contained in 12,000μ constrains the size of b.The far field pattern of the diffraction grating is$I = {{I_{o}( \frac{\,_{\sin}\beta}{\beta} )}^{2}{( \frac{{\,_{\sin}N}\quad \alpha}{\alpha} )^{2}.}}$

[0101] N=number of lines illuminated,$\beta = {\frac{\pi \quad b}{\lambda}\sin \quad \theta_{b}}$

[0102] and$\alpha = {\frac{\pi \quad d}{\lambda}\sin \quad {\theta_{b}.}}$

[0103] Spread sheet calculations show that b≦5.5λ (or b≦8.5μ), isnecessary to make the spectral image>12,000μ at its 90% intensity point.To minimize loss, i.e., maintain adequate efficiency, B>2d. Thus,2λ<b<5.5λ. (Condition A).

[0104] In littrow mode, the angular dispersion is:$( \frac{\theta}{\lambda} ) = {{\frac{m}{d\quad \cos \quad \theta_{b}}\quad {or}\quad ( \frac{\theta}{\lambda} )} = \frac{m}{b}}$${\Delta \quad x\quad ( {{linear}\quad {separation}} )} = {{( {\Delta \quad \theta} )( {f\quad l} )( {\Delta \quad \lambda} )} = {( \frac{m}{b} ){( {\Delta \quad \lambda} ) \cdot ( {f\quad l} )}}}$${80µ} < {\frac{m}{b}( {4 \times 10^{- 4}µ} )( {1.2 \times 10^{5}µ} )}$$m > \frac{1.6b_{µ}}{{.6}_{µ}} > {1.6b_{µ}}$

[0105] However, for OC192, dispersion must be constrained to contain the0.08 nm channel width in a 10μ core, so that m<3.34μ.

[0106] Thus, 1.67b<m<3.34b (Condition B).

[0107] The desired resolution$(R) = {\frac{\lambda}{\Delta \quad \lambda} = {N \cdot {m.}}}$

[0108] Here, λ=1550 nm and Δλ=0.08 nm, yielding a required resolutionR=19,375 or approximately 20,000. Assuming a beam size at the grating of2.1 cm (based upon a fl=124 cm and 10° divergence):${N = \frac{p(2.1)}{\cos \quad \theta_{b}}},{p = {{{lines}/{cm}} = \frac{1}{d}}}$

[0109] Thus,${{20,000} < {\frac{2.1 \times 10^{- 2}\quad {cm}}{d\quad \cos \quad \theta} \cdot m}} = {\frac{2.1^{- 2}\quad {cm}}{b}m}$

[0110] or b<1.05 m (Condition C).

[0111] To align the order m with the diffraction peak in littrow mode,we know ${a = \frac{m\quad \lambda}{2}},$

[0112] or a must have the values: $ {a = \quad \begin{matrix}{{3.88µ\quad {at}\quad m} = 5} \\{{4.65µ\quad {at}\quad m} = 6} \\{{5.43µ\quad {at}\quad m} = 7}\end{matrix}} \} \quad ( {{Condition}\quad D} )$

[0113] Only as θ_(b) increases to greater than 45° is it possible forconditions A and D to be satisfied. Assuming θ_(b)=60°, and m=5,

[0114] a=3.38μ

[0115] b=2.24μ

[0116] d=4.48μ

[0117] All of conditions A-D are satisfied.

[0118] Selection of the precise groove density and blaze angle are alsoaffected by the polarization dependent loss and manufacturingconstraints. For the embodiment illustrated in FIG. 4 use of aninterferometrically controlled ruling engine to machine the line gratingdrove the selection of a line density evenly divisible by 3600.Considering these various factors led to selection of groove densityd=171.4 grooves/mm and m=5. This leads to a=3.88μ, b=3.55μ, and acorresponding blaze angle of 52.6° for this example. However, thismethodology shows that for a focal length between 30-125 mm and an orderof 5-7, potential blaze angles range between 51° and 53° and the groovedensity carries between 50 and 300 grooves/mm to provide linear channelseparation of between 40-125 microns and an angular dispersion of theechelle of between 0.091 and 0.11 degrees/nm.

[0119] In the example of FIG. 4, the echelle grating has a groovedensity of 171.4 grooves/mm and a blaze angle of 52.6°. The echelle maybe formed from one of several known methods. For example, it may beformed from an epoxy layer deposited on a glass substrate into which amaster die defining the steps is pressed. The steps are then coated witha highly reflective material such as gold. The steps may also beprecision machined directly into a glass or silicon substance and thencoated with a reflective material. A further option is the use ofphotolithographic techniques described in McMahon, U.S. Pat. No.4,736,360, the contents of which are hereby expressly incorporated byreference in its entirety.

[0120] The lens 18 could be a graded index (GRIN) optic with sphericalsurfaces or a compound lens with one or more surfaces that might not bespherical (aspheric). The use of lenses or a single lens to collimatethe beam and focus the dispersed light limits spherical aberrations orcoma resulting from the use of front surface reflectors that require theoptical rays to traverse the system in a off-axis geometry. A first typeof potential lens uses a radially graded refractive index to achievenear-diffraction limited imaging of off-axis rays. A second type of lensactually consists of at least two individual pieces cemented together(doublet). Another option uses three individual lens pieces (triplet).These pieces may individually have spherical surfaces, or if requiredfor correction of certain types of aberration, aspheric surfaces can beutilized. In this case, the lens would be referred to as an asphericdoublet or triplet.

[0121] In the example illustrated in FIG. 4, the lens 18 is an asphericsinglet of a 25.4 mm diameter having a spherical surface 26 with aradius of curvature of 373.94 mm and an aspheric surface 28 with aradius of curvature of 75 mm and a conic constant of ˜0.875. The averagefocal length in the 1520-1580 nm wavelength range is 125.01 nm. Thus,the distance A from the center of the spheric surface to the emittingend of the input and output fibers 14, 16 is about 125 mm. The averagedistance between the aspheric surface 28 and the center of the surfaceof the grating 20 is about 43.2 mm.

[0122] In the pigtail 12 of FIG. 1, the input and output fibersterminate in the same plane. This is also the case with the exampleillustrated in FIG. 4. In some configurations, however, the inlet 14 andoutlet fibers 16 are on slightly different axes and do not terminate inthe same plane. The fibers 14, 16 of the pigtail are precisely locatedby being fit into a template 34 illustrated schematically in FIG. 5. Thetemplate 34 has a plurality of parallel v-shaped grooves 36. Thetemplate and v-shaped grooves are preferably formed by etching thegrooves 36 into a silicon substrate. In the example in FIG. 4, thegrooves of the template are spaced 80μ.

[0123] The example configuration of FIG. 4 is shown in perspective viewin FIG. 6. To facilitate alignment, the pigtail 12, the lens 18 and thegrating 20 have limited freedom of movement in multiple directions. Oncethey are moved into position, they are secured in place by clamps or asuitable bonding agent. The lens 18 is held stationary. The pigtail 12is movable by translation along the x, y and z axes. The input andoutput fibers can be moved independently along the x axis. The echellegrating 20 is fixed against translational movement except along the zaxis. It can be rotated about each of the x, y and z axes. Otherpossible combinations of element movement may also yield suitablealignment.

[0124] The dimensions and performance criterion of the DWDM device 10 ofFIG. 4 are summarized as follows:

[0125] Fibers: SM-1250 (Fibercore) outer diameter

[0126] Outer diameter 80μ

[0127] Core diameter 8.3μ

[0128] f Number 4-8

[0129] Lens: Aspheric singlet

[0130] Average focal length (fl)=125

[0131] Optical Signal: λ=1528-1565 nm channel spacing=0.4 nm

[0132] Grating:

[0133] d=5.83μ

[0134] θ_(b)=52.6°

[0135] order=6

[0136] System Performance:

[0137] D (linear separation)=80μ

[0138] Resolution (R)=20,000

[0139] Efficiency=75%

[0140] As an alternative to the use of a littrow configuration as wellas the use of collimating lenses, concave mirrors may be used forcollimating and focusing the incident beam. A first alternate embodimentof a concave mirror dense wavelength (de)multiplexer 40 is shownschematically in FIG. 7. Single mode input fiber 42 emits a divergentincident beam 44 consisting of multiplexed channels onto the surface ofa collimating/focusing concave mirror 46. The collimated beam 48 is thendirected in an off-axis manner to the surface of an echelle grating 50.The echelle grating disperses the channels according to their wavelengthin the manner discussed above with respect to FIGS. 1 and 4 and thedispersed channels 52 are reflected off axis off the front surface ofthe concave collimating/focusing mirror 54. The collimating/focusingmirror 54 then focuses and reflects the various channels to acorresponding fiber of an output fiber array 56. As alluded to abovewith respect to the discussion of the embodiments of FIGS. 1 and 4, useof surface reflecting optics such as the collimating mirror 46 and theconcave focusing mirror 54 requires that the optical beams traverse thesystem in an off-axis geometry which creates significant aberrations(spherical aberrations and coma) that significantly limit theperformance of the system. However, the use of the front surfacereflecting optics has the potential of facilitating a more compact formfactor than is possible with littrow configurations using a singleoptical lens. As should be readily apparent, combinations of frontsurface reflecting optics and lenses can be used in non-littrowconfigurations where necessary to balance form factor minimizationrequirements and optical aberrations.

[0141] A second alternate embodiment 60 is illustrated in FIG. 8 whichis a schematic representation of an echelle grating (de)multiplexerusing a prism in combination with front surface optical mirrors. In thisembodiment, light from a single mode input fiber 62 is directed off acollimating/focusing mirror 64 and the collimated beam 66 is directedthrough prism 68. The prism 68 provides for wavelength dispersion in ahorizontal direction as indicated by the beams 70. These horizontallydispersed beams 70 are directed off the echelle grating 72 which in turndiffracts the beams 70 in an orthogonal dimension and directs thesediffracted beams off the front surface of the concavecollimating/focusing mirror 74. A two dimensional output fiber array 76receives the focused beams from the collimating/focusing mirror 74. Theuse of the prism 68 in combination with the echelle grating 72 providesa two dimensional array of wavelength dispersion and may thereforefacilitate detector arrays of shorter length as may be desirable incertain applications.

[0142]FIG. 9 is a schematic representation of a third alternateembodiment 80 using a single concave mirror as both a collimating andfocusing optic along the optical axis. In this embodiment, input fiber82 directs a beam consisting of multiplexed channels to the surface ofthe concave mirror 84. A collimated beam 86 is reflected off the echellegrating 88 which diffracts the multiplexed signal in the mannerdiscussed above. The demultiplexed channels are then reflected off thesurface of the concave mirror 84 and directed into the array of outputfibers 92. While the embodiment 80 contemplates the mirror 84 beingspherical and therefore having a constant diameter of, for example 25cm, a slightly parabolic or aspheric mirror may be used to improve imagequality, if necessary.

[0143]FIG. 10 is a fourth alternate embodiment 100 using an off-axisparabolic mirror as the collimating/focusing optic. In this embodiment,multiplexed light from the input fiber 102 is directed off the frontsurface of an off-axis parabolic mirror 104 which in turn directs acollimated beam of light 106 off the surface of an echelle grating 108.The multiplexed light is reflected off the surface of the echellegrating 108 back to the surface of the off-axis parabolic mirror 104 anddispersed to respective output fibers 106. In this embodiment, theechelle grating is in near-littrow configuration, thereby directinglight back to the output fibers 106.

[0144] A fifth alternate embodiment illustrated in FIG. 11 uses aconcave echelle grating 107 configured to be the optic which collimatesand focuses the incoming beam. This embodiment eliminates the need forthe collimating/focusing lenses or concave mirrors of alternateembodiments one-four.

[0145] Various modifications can be provided to the basic echellegrating (de)multiplexer structures illustrated schematically in FIGS.1-11 to further increase the channel carrying capacity of single modeoptical fibers. As alluded to above, it is foreseeable in the futurethat advancements in optical amplifier technology will enable bandwidthin excess of the current 60-80 nm bandwidth used in opticalcommunication. Such broad bandwidths tax the ability of an echellegrating DWDM to effectively multiplex and demultiplex the entirebandwidth, particularly in the frequencies at the edge of this broadband. Accordingly, it would be useful and desirable to use a network ofechelle grating DWDM devices with each device optimized tomultiplex/demultiplex light in a portion of the broad spectral range.For example, assuming future amplifier technologies enable bandwidths onthe order of 120-180 nm, each echelle grating DWDM could be optimized tofunction with a portion, for example ½, of the bandwidth, 60-90 nm.

[0146]FIG. 12 illustrates schematically an apparatus 110 for dividing abroad bandwidth for (de)multiplexer. The apparatus 110 consists of aninput fiber 112, a high pass thin film filter 114, a first focusing lens116, a second focusing lens 118, a first echelle grating DWDM device 120and a second echelle DWDM device 122.

[0147] By way of example, the operation of the apparatus for dividingbroad band signals 110 will be discussed in terms of a demultiplexer. Aswith other embodiments of this invention, the apparatus may likewisefunction as a multiplexer simply by reversing the direction of lightpropagation. A multiplexed beam 124 emitted from the input fiber 112 isdirected onto the high pass thin film filter 114. The high pass thinfilm filter has a design cut off wavelength that reflects the lower halfof the wavelength range toward the first echelle grating DWDM 120. Theupper half of the wavelength range passes through the filter 114 to thesecond echelle DWDM device 122. In this example, the input wavelength isin the range of 1460-1580 nm. The high pass thin film filter is designedto cut the band at 1520 nm. Thus, a wavelength range of 1460-1520 nm isdirected toward the first echelle grating DWDM and a wavelength band of1520-1580 nm is directed toward a second echelle grating DWDM device.The signal directed toward the first echelle grating DWDM is opticallycoupled to the first focusing lens 116 which directs the lowerwavelength beam as an input to the first echelle grating DWDM. In a likemanner, the upper wavelength beam 128 is optically coupled to the secondfocusing lens 118 which focuses the upper wavelength beam 128 as aninput beam to the second echelle DWDM device 122.

[0148] The present example contemplates the use of a high pass thin filmfilter 114. However, other waveband dividing elements could be usedinstead, including devices using fiber Bragg gratings.

[0149] The first and second echelle grating DWDM devices 120, 122 of thepresent invention could have any of the configurations discussed abovewith regard to FIGS. 1-11. The use of the echelle DWDM devices fordemultiplexing the split wavelength bands provide the many advantagesdiscussed above with regard to the embodiments illustrated in FIGS.1-11. However, the present invention could be practiced with other DWDMdevices such as fiber Bragg grating devices, integrated waveguide arraysor the like. With an echelle spectrograph permitting wavelength spacingof 0.4 nm, a device for providing a total wavelength range of 120 nmwill allow up to 300 channels to be demultiplexed from a single fiber.Furthermore, this system is scalable. FIG. 13 illustrates schematicallyhow an input bandwidth of 1460-1700 nm can be divided using threewaveband dividing elements to four 60 nm bandwidth beams each of whichcan be input into an optimized echelle grating DWDM device. Such adevice is capable of accommodating a total waveband of 240 nm andassuming a wavelength spacing of 0.4 nm, a total channel count of 600.

[0150] The bulk optic echelle DWDM of the present invention is able tosimultaneously demultiplex signals from a number of input fibers. Ineach of the echelle grating DWDM devices illustrated in FIGS. 1-7 and9-11 above, light is spacially resolved in only one dimension,vertically in a direction transverse the dispersion direction. As aresult, input fibers can be vertically stacked in a linear array and acorresponding two dimensional array of output fibers can be provided forreceiving demultiplexed signals from the various input fibers. Thisconcept is illustrated schematically in FIG. 14. FIG. 14 is an elevationview of a pigtail harness 140 from the direction of thecollimating/focusing optic. First, second and third input fibers 142,144, 146 lying in a vertical linear array are optically coupled tofirst, second and third horizontal output rows 148, 150, 152,respectively. Thus, a one dimensional input array produces atwo-dimensional output array. While the present example is limited tothree input fibers 142, 144, 146 and only nine output fibers in theoutput first, second and third output rows 148, 150, 152, the actualnumber of output fibers will correspond to the number of input channelsand will be a function of the channel separation and input bandwidth,and may easily exceed 90 output fibers per output fiber row. Each outputfiber has a core center, and the output fiber core centers are spaced adistance equal to the linear separation of the grating at the devicefocal length. Further, the number of corresponding input and outputarrays may be greater than three and is largely a function of externalfactors such as the space available for the pigtail harness 140. Asshould be appreciated, this configuration allows a single demultiplexerto demultiplex channels from a number of input fibers, therebyminimizing the number of echelle grating DWDM devices required for amultiple input fiber optical system. This further illustrates theflexibility and scalability of the echelle grating DWDM devices inaccordance with the invention.

[0151]FIG. 15 is a schematic representation of a preferred embodiment ofa stacked input bulk optic echelle DWDM device 160. Input beam λ¹ ₁₋₁₀from input fiber 142 is directed to the collimating/focusing optic 162and a collimated beam is then directed off the reflective surface of thereflective echelle grating 164. The diffracted channels λ¹ ₁, λ¹ ₂ thenreturn through the collimating/focusing optic 162 and are dispersed tothe fibers comprising the first output row 148 as illustrated by λ¹ ₁.The collimating/focusing optic has an optical axis 166 and the inputfiber 142 and the output row 148 are equally spaced from the opticalaxis 166 of the collimating/focusing optic in the vertical direction. Ina like manner, a multiplexed input beam λ² _(1−n) is emitted from theinput fiber 144 and its various channels λ² ₁, λ² ₂ are diffracted tothe second horizontal output row 150. With respect to each of outputrows 148 and 150, the centers of the optical fibers in the row are eachspaced a distance from the centers of adjacent optical fibers in the rowequal to the channel separation of the echelle grating 164 at the focallength of the collimating/focusing optic 162. The propagating ends ofthe output fibers as well as the propagating ends of the input fibersall lie in a plane spaced the focal length of the collimating/focusingoptic from the collimating/focusing optic.

[0152] The echelle grating DWDM devices in accordance with the presentinvention provide for dense channel spacing (0.4 mn) over a givenbandwidth, thereby maximizing the number of channels that can be carriedby a single fiber for a given bandwidth. By careful selection of theechelle grating blaze angle and step spacing, the channels may bemultiplexed/demultiplexed at high resolutions and high efficiencies.Further, use of the echelle grating enables a smaller form factorbecause the angular diffraction allows for shorter focal lengths betweenthe focusing lens and the input/output fibers. The use of bulk opticalelements provides a system which is easy to manufacture, highly reliableand scalable. Further embodiments of the invention including the use ofa waveband dividing element such as a thin film high pass filter allowsextremely broad bands of signals to be divided and simultaneouslymultiplexed or demultiplexed in parallel. Because the device disperseslight in a single linear dimension, a plurality of input fibers can bestacked so that each bulk optic echelle grating DWDM device canaccommodate multiple input fibers.

[0153]FIG. 17 is a schematic representation of a telecentric(de)multiplexer 230 in accordance with the present invention. Theelements of FIG. 17 which are identical to those of FIG. 16 will havethe same reference number followed by a prime (e.g. 16′). As with FIG.16, only a few equally spaced single channel fibers are included forsimplicity. In the embodiment of FIG. 17, the grating 18′ is situated sothat center 231 of its reflective surface is located at the focaldistance F of lens 16′. The array of single channel fibers 220′ issituated with the faces 228′ at or near the focal plane 223 located atthe focal length F from the lens 216′. As used herein, “at or near thefocal plane” means located physically at or close enough to the focalplane that any performance degradation resulting from not being at thefocal plane is inconsequential. Because the grating 218′ is reflective,this configuration produces a 4F configuration as between the microlensarray 220′, the lens 216′ and the grating 218′. The 4F configurationresults in a telecentric optical beam being propagated to the distalfiber 222′. In other words, the axis 226′ is substantially normal to theface 228′ of the distal fiber 222′. Likewise, the other single channelfibers 220 receive a telecentric beam; that is, the axis of theirincident beams are essentially perpendicular to their face.

[0154]FIG. 18 is another embodiment of a (de)multiplexer 240. Elementsof FIG. 18 which are identical to those of FIG. 16 have the samereference number followed by a double prime (e.g., 16″). Like FIG. 16,only a few equally spaced single channel fibers are included forsimplicity. A microlens array 242 is situated at or near the focal plane243 of the lens 16″. The grating 218″ is situated so that the center 244of its reflective surface is located at the focal distance F of lens216″ opposite the microlens array 242. Because the grating 218″ isreflective, this configuration has essentially a 4F configuration asbetween the microlens array 242, the lens 216″ and the grating 218,″resulting in a telecentric (de)multiplexer.

[0155] Each lens 246 of the microlens array has a diameter ‘D’ which issignificantly greater than the diameter ‘d’ of the core 248 of thesingle channel fibers. As known in the art (see Chamberlin and Hill,“Designs for High Channel Density Single-ModeWavelength-Division-Multiplexers,” Proceedings of the SPIE, Vol. 839(p.60-66) (1987)) this has the effect of broadening the pass band so asto improve coupling with the fibers. In addition, the 4F configurationillustrated in FIG. 18 makes the (de)multiplexer telecentric. As aresult, light from the microlens array 246 strikes the face 228″ of thefiber cores essentially perpendicular to the plane of the face. This notonly improves the coupling efficiency, but allows for greater tolerancesin alignment of the optical axis of the fiber cores with the opticalaxis of the microlenses making up the microlens array. As a result,limited fabrication and environmentally caused differences between pitch‘P’ between the optical axis 250 of adjacent microlenses and the pitch‘p’ between the optical axis 252 of corresponding adjacent singlechannel fibers 220″ will not affect the performance of the device.

[0156] One important benefit of this configuration is that the devicesize, that is, length along the optical axis, can be reducedsignificantly because the beam divergence of the input light has greatlydecreased by the microlens array 242. As a result, free spacediffraction grating based (de)multiplexers can be made comparable inphysical dimensions to that based on the planar Planaz Lightwavecircuits.

[0157] Each of the microlenses 246 which constitute the microlens array242 is a conventional plano-convex spherical refractive lens. The inputplane of the fiber faces 228″ can be situated such that the focal pointof the lenses 246 is at the face 228″ as illustrated at 254 or withinthe fiber core some distance as illustrated at 256 and 258. This featurefurther gives adjustability on pass band broadening. The tunability ofthe pass-band broadening permits optimizing of the pass band withoutintroducing unacceptable cross talk. The telecentricity of the 4F systemfurther increases the range of tunability. Finally, the feature providesfor greater tolerances in alignment of the fibers along the optical axisof the (de)multiplexer.

[0158] In the preferred embodiment, the grating 218″ is an echellegrating in accordance with the invention described above with referenceto FIGS. 1-6. Because of the enhanced angular dispersion of the echellegrating, the telecentricity of the configurations illustrated in FIGS.17 and 18 is of enhanced significance. That is, the 4F configurationprovides telecentricity which provides an efficient coupling with thefiber cores which cannot be achieved absent telecentricity because ofthe steep angular dispersion of the echelle gratings, particularly atthe extreme ends of the fiber array. As further discussed above, thetelecentricity further allows finite differences between the pitch ‘P’between adjacent lenses of the microlens array and the pitch ‘p’ betweenthe optical axis of corresponding adjacent fibers. Thus, the pitch canbe intentionally offset and/or the pitches can be set to much greaterfabrication tolerances than prior art devices. Furthermore, use ofrefractive plano-convex spherical lenses 246 in the microlens array ofthe embodiment of FIG. 18 simplifies the fabrication of the microlensarray and also allows for the focal point to extend varying distanceswithin the core of the single channel fibers without effecting couplingefficiency, a significant advantage over the teachings of Martin. Thisfurther provides for a greater tolerance in alignment along the opticalaxis of the (de)multiplexer and tunability in pass band broadening.Moreover, the telecentricity enhances the range within which the passband can be tuned.

[0159] The specific design parameters such as the focal length anddiameter of the imaging lens and the microlenses will be a factor of thesize requirements, grating properties, input channel spacing, singlechannel core diameter and core spacing, and wavelength of the opticalsignal among other factors known in the art. Representative microlensesused with an echelle grating of the type described with respect to FIGS.1-6 with an optical signal having a wavelength of 1528-1565 nm and a 0.8nm channel spacing would be a plano-convex lens having a focal length onthe order of 300 microns and a diameter of about 80 microns and arepresentative imaging lens would be a spherical cemeted doublet with afocal length of about 65 mm and a diameter of about 10 mm.

1. An apparatus for use in optical communication systems to multiplex ordemultiplex an optical signal, the apparatus comprising: a plurality ofoptical waveguides aligned generally along the same optical axis eachhaving a propagating end; a reflective grating optically coupled to theplurality of optical waveguides along the optical axis having a surfacereceiving an optical signal emitted from the propagating end of at leastone of the optical wavegnides and diffracting the optical signal(s) intothe propagating end of at least one other of the optical waveguides; anda collimating/focusing optic having a select focal length opticallycoupled between the propagating ends of the plurality of opticalwaveguides and the reflective grating along the optical axis, thecollimating/focusing optic being positioned relative to the propagatingends of the plurality of optical waveguides and the reflective gratingto propagate a telecentric optical beam(s) into the at least one otherof the optical waveguides.
 2. The apparatus of claim 1 wherein thecollimating/focusing optic is positioned with the propagating ends ofthe waveguides at about the select focal length from thecollimating/focusing optic and a point on the surface of the reflectiveechelle grating being spaced the select focal length from thecollimating/focusing optic.
 3. The apparatus of claim 1 wherein theplurality of optical waveguides comprise at least one multiplexwaveguide propagating a plurality of multiplexed channels and the othersof the optical waveguides are single channel waveguides propagatingsingle channels, the single channel optical waveguides being in a lineararray, the apparatus further comprising: a linear array of microlenseswith a microlens optically coupled to each single channel waveguide, themicrolens array residing between the propagating end of the singlechannel waveguides and the collimating/focusing optic.
 4. The apparatusof claim 3 wherein the linear array of microlenses is located at about afocal plane spaced about the select focal length from thecollimating/focusing optic.
 5. The apparatus of claim 1 wherein thereflective grating is an echelle grating having a groove spacing ofbetween about 50-300 grooves per millimeter and a blaze angle of betweenabout 51-53 degrees.
 6. The apparatus of claim 1 wherein the reflectivegrating is an echelle grating having a groove spacing of between about50-300 grooves per millimeter and a blaze angle providing an angulardispersion of between about 0.091 and 0.11 degrees per nanometer for aselect order of diffraction of between 4-7.
 7. The apparatus of claim 4wherein a focal point of at least one microlens is in a correspondingwaveguide.
 8. The apparatus of claim 4 wherein the focal point of atleast one microlens is at the propagating end of a correspondingwaveguide.
 9. An apparatus for use in optical communication systems tomultiplex or demultiplex an optical signal, the apparatus comprising: aplurality of optical waveguides aligned generally along the same opticalaxis each having a propagating end, the plurality of optical waveguidesincluding at least one multiplex waveguide propagating a plurality ofmultiplexed channels and the others of the optical waveguides beingsingle channel wavegnides propagating single channels; a reflectivegrating optically in optically coupled with the plurality of opticalwaveguides along the optical axis having a reflective surface receivingan optical signal emitted from at least one of the optical waveguidesand diffracting the optical signal(s) into at least one other of theoptical waveguides; a collimating/focusing optic having a select focallength optically coupled between the propagating ends of the pluralityof optical waveguides and the reflective surface of the echelle gratingalong the optical axis, the reflective surface of the echelle gratingbeing spaced about the select focal length from the collimating/focusingoptic; and an array of microlenses residing in optical communicationbetween the propagating end of the single channel waveguides and thecollimating/focusing optic, the array of microlenses being positionedrelative to the collimating/focusing optic to propagate a telecentricoptical beam(s) into the at least one other of the optical waveguides.10. The apparatus of claim 9 wherein the single channel opticalwaveguides are in a linear array and the array of microlenses is alinear array.
 11. The apparatus of claim 9 wherein the reflectivegrating is an echelle grating having a groove spacing of between about50-300 grooves per millimeter and a blaze angle of between about 51-53degrees.
 12. The apparatus of claim 9 wherein the reflective grating isan echelle grating having a groove spacing of between about 50-300grooves per millimeter and a blaze angle providing an angular dispersionof between about 0.091 and 0.11 degrees per nanometer for a select orderof diffraction of between 4-7.
 13. The apparatus of claim 9 wherein thefocal point of at least one microlens is in a corresponding waveguide.14. The apparatus of claim 9 wherein the focal point of at least onemicrolens is at the propagating end of a corresponding waveguide. 15.The apparatus of claim 9 wherein the reflective grating is spaced adistance from the collimating/focusing lens such that a point on areflective surface of the grating is spaced the select focal length fromthe collimating/focusing lens.
 16. The apparatus of claim 15 wherein thepoint on the reflective surface of the grating is at about the center ofthe reflective surface of the grating.
 17. The apparatus of claim 9wherein the array of microlenses is positioned at about a focal plane ofthe collimating/focusing optic spaced the select focal length from thecollimating/focusing optic.